1. Field Of The Invention
The present invention relates to diffraction-grating spectrometers and monochromators and, more particularly, to a Littrow-type diffraction grating spectrometer.
2. Background Information
It is known in the art to use various optical modules that are assembled into a desired configuration to perform a specified optical function. Such configuration may take the form of a scientific instrument, or may find employment in a spectroscopy application. Modules that in turn comprise a number of devices for performing optical functions are also known. It is usually advantageous to make each module as compact as possible.
A spectroscope, an instrument which produces a spectrum, is one particularly useful example of such an optical instrument. Another is a spectrograph which is a spectroscope provided with a recording device, or other light-capture means, or the like to receive and record or otherwise process the spectrum generated. A spectrometer is a spectrograph enhanced with means to quantify the output, for example a scale to measure particular wavelengths, or a detector to determine intensity, at one or more wavelengths. The present invention relates particularly to spectrometers, and that term will be used hereinafter. However it will be understood that the novel optics described herein can be employed in spectroscopes or spectrographs for applications where recording, quantification or similar capabilities are not required and the invention extends to such novel spectroscopic and spectrographic applications. To the extent that the invention may be applied to output a single spectral band or to provide a scanned output comprising a series of individual spectral bands, the term xe2x80x9cspectrometerxe2x80x9d should also be understood to include monochromators.
Diffraction grating spectrographs use one or more diffraction gratings to diffract input light into a spectrum of specific wavelengths or spectral bands. In a typical configuration, spectrographs are designed to select a single wavelength, or a narrow spectral band from the input light, for examination or recordal.
In one known embodiment of spectrometer employing a planar diffraction grating, a concave mirror is illuminated by a point source whose spectrographic composition is to be analyzed. The light from the point source is collimated by the concave mirror to form a parallel bundle of rays, which are caused to fall upon the surface of a planar diffraction grating. This concave mirror is known as a collimator in a typical spectrometer instrument. Because the planar diffraction grating has a number of grooves etched in its surface, light falling on the surface of the diffraction grating is diffracted, that is, reflected an angle which is a function of the wavelength of the light. If the input light source comprises a number of wavelengths, the result is that light of different wavelengths will be diffracted, or reflected, at an angle which is a function of wavelength.
The diffracted light may then be received by a second concave mirror which focuses the diffracted light to form an image of the point source under analysis. However, because light of different wavelengths has been diffracted at different angles, the point source is imaged by the second concave mirror, also known as a focusing mirror, at different points for different wavelengths. Accordingly, it is possible to select out individual wavelengths, or more precisely a narrow region of the spectrum, or spectral band, consisting essentially of a single wavelength, to measure the intensity of the same and to utilize this information, for example for elemental analysis of an emissive source material.
Spectrometric elemental analysis of samples has many industrial uses. For example, in the case of the analysis of industrial slag, such as might be obtained from crucible of molten metal in a steel furnace, the slag may be put into a plasma, excited and the emission spectrum analyzed and measured with a spectrometer. The wavelengths appearing in the plasma emission band indicate the nature and quantity of the impurities in the slag, enabling plant operators to adjust production parameters to achieve a desired product.
While the above discussion has centered on spectrometer devices using mirrors, and such devices are usually preferred because of the quality of imaging using mirrors, it is possible to construct devices using focusing lenses, such as convex lenses or compound multielement lenses having an overall convex optical characteristic. In principle, it is also possible to combine lenses and mirrors in an instrument.
It is also noted that diffraction gratings in spectrometers may be either classical mechanically ruled diffraction gratings of the type invented and made by applicant""s assignee at the beginning of the 1800""s, or holographic diffraction gratings of the type manufactured by applicant""s assignee since the 1960""s.
It is also known that spectrometers may be constructed using concave diffraction gratings, such as concave holographic diffraction gratings of the type invented by Flamand in the late 1960""s working at the applicant company as illustrated by his U.S. Pat. No. 3,628,849.
A Littrow-mounted system is a relatively common method of utilizing large plane reflection gratings, providing simplicity and good optical quality arising from the use of a single nmirror to perform both collimating and focusing functions. Moreover, in this configuration, the collimating and focusing functions are both performed in the same geometric space, resulting in efficient use of that space. In a typical Littrow setup, a mirror delivers parallel incident light from an input point source to the grating, and focuses diffracted light received from the grating to an output point often proximate the input point source. In such devices, a single mirror acts as both collimator and focusing element at once, minimizing the number of optical elements required.
In addition to its simplicity, employing the Littrow configuration is particularly desirable for its high quality output. Because the input and output light beams traverse the same optical path, in opposite directions, optical aberrations in the collimating and focusing components are auto-corrected, or self compensating, so that image quality is diffraction limited, i.e. limited by the physical properties of the optical system not by the deficiencies of the optics.
Referring now to FIG. 1, one embodiment of a prior art, Littrow-mounted, plane grating spectrometer 1 is shown schematically. Spectrometer 1 employs a rotatably mounted diffraction grating 7 having an inlet aperture, or slit 2, providing a point or line light source and admits light from a source 3 in the direction of arrows 4 toward a concave focusing mirror 5. Light reflected from mirror 5 is focused to travel in a collimated beam in the direction of arrows 6 which then strikes diffraction grating 7.
When the collimated light 6 strikes grating 7 it is diffracted at an angle which varies as a function of wavelength. Accordingly, grating 7 is rotated to variably select one wavelength from a number of wavelengths, as desired, or to scan through the spectrum of available wavelengths. Thus, a selected wavelength of light 8 is reflected back, at the Littrow angle, to travel in a parallel beam in the direction of arrows 9, oppositely to the direction of arrows 6. The returned selected wavelength of light traveling in directions 9 strikes concave mirror 5 and is focused to aperture 2.
An image can then be formed on a detector placed at aperture 2, if desired, or otherwise recorded or processed and quantified, if desired. From a practical standpoint, placement of the detector at a point on a slit may be undesirable. Therefore, the detector may be slightly offset, and the system tuned to select the desired wavelength, or other desired wavelengths, by rotation of grating 7 to an angular position that results in the imaging of that wavelength on the detector. As described above, a high quality, diffraction-limited output can be obtained. The instrument-shown, can function as a monochromator, if desired.
A particular drawback of such conventional Littrow-mounted grating configurations is the difficulty and expense of providing grating 7 with a central optical opening 2. Another drawback is that undue stray light may be returned to aperture 2 by mirror 5. It would be desirable to provide a spectrometer or comparable optical system, which did not suffer from these drawbacks.
The invention, as claimed, is intended to provide a remedy. It solves the problem of providing a Littrow-mounted grating spectrometer for diffracting a light sample and selecting out a particular wavelength or spectral band without need for an optical opening or aperture in the grating.
To solve this and other problems, the invention provides, in one aspect a spectrometer capable of outputting light in a selected spectral band from a sample light input, the spectrometer comprising:
a) a planar diffraction grating capable of receiving a collimated beam of the sample light, input along an incident path at an incident angle, and of diffracting the received incident light to provide a diffracted light output of the selected spectral band, along the incident path in the opposite direction to the incident light; and
b) a planar mirror disposed to direct the incident beam to the planar grating and receive the diffracted light from the diffraction grating on a light path that is a reflected path with respect to the planar mirror;
wherein the planar mirror can communicate optically with a light sample source and a light output. Preferably, the geometry is such that the planar mirror directs the incident beam at the Littrow angle for the selected spectral band. Use of a planar mirror to provide two-way light communication with the grating enables a compact instrument to be realized by employing suitable input-output optics.
The input-output optics can comprise a concave mirror in optical communication with the planar mirror, the light sample source and the light output. Preferably, the concave mirror provides the collimated sample light beam to the planar mirror along a planar mirror incident path and receives the diffracted light output along the same planar mirror incident path.
In one particularly preferred embodiment, a light aperture, which serves for both the input and the output light, is provided in the form of an optical opening through the planar mirror.
The light aperture can both admit light from the sample source and provide an exit for the light output. With particular advantage, the light aperture is disposed for source light received into the spectrometer through the light aperture to be collimated by the concave mirror and for the diffracted light output to be focused by the concave mirror to the light aperture, providing a Littrow mounting. For convenience, the planar mirror and the concave mirror are assumed to be disposed on a horizontal optical axis, although, the optical instruments of the invention can of course have any desired spatial orientation.
In effect, with such an arrangement, the optically apertured planar mirror folds the Littrow-collimated input-output light beam enabling the grating to be disposed geometrically above or alongside the optical pathway between the planar mirror and the concave mirror. The planar grating can be disposed to be rotatable through an orientation perpendicular to the optical axis, and may thus have significant length, enhancing image intensity, without significantly increasing the length or other dimension of the instrument.
By providing the spectrometer""s input and output through the optical aperture in the planar mirror and disposing the concave mirror with its focal point in the vicinity of the optical aperture, the divergent-convergent light paths to and from the concave mirror are effectively laterally contained between the two optical elements, further enhancing the compactness of the inventive spectrometer.
The invention furthermore provides a method of optically selecting a spectral band from a sample light received through a light aperture located in and surrounded by a planar mirror, the method comprising:
a) collimating divergent sample light received from the input aperture with a concave mirror;
b) reflecting the collimated sample light with the planar mirror to a diffraction grating at the Littrow angle for the spectral band;
c) reflecting the diffracted spectral band with the planar mirror to the concave mirror; and
d) focusing the diffracted spectral band to the output aperture.
In another aspect, the invention provides a spectrometer for producing a diffracted light output in a selected wavelength range from a sample light input, the spectrometer comprising:
a) a light aperture providing an optical inlet to the spectrometer for sample light to travel on an input light path and an optical outlet for output light traveling on an output light path;
b) a concave mirror to collimate the input light from the light aperture along an optical axis and focus the output light to the light aperture,
c) a planar mirror angled across the optical axis to reflect the input light received from the concave mirror to an optical processing unit for processing the input light to provide the output light, the input light and the output light traveling to the optical processing unit along the same path, in opposite directions.
In this aspect, the invention provides a compact aberration-corrected optical input-output unit or system which can be used with a variety of optical processing units that are operable with collimated input and output light traveling in opposite directions on the same path.